The reason for supplying descriptions of those methods is to create a general contract. That means if a method in an interface says that it makes smoke come out of your computer, then all implementing classes must make smoke come out of the computer, too. If you look at the add method in that interface you linked to, and then at the add method in the subinterface
List, you will see a difference. Collections provides a very vague general contract which is greatly refined by List. That means the Collection version can be used in a set, too.
That means your implementing methods may cause white smoke or black smoke to come from your computer whilst still complying with their superinterfaces' general contracts